Control of trains and their movements along defined rails has been a priority since the inception of railroads. Safety concerns are many including concerns about collisions, derailments, overtaking and colliding with stopped or slower moving trains, head on collisions when trains are traveling on the same rail in opposite directions, improper switch orientation causing trains to enter onto and travel on wrong tracks creating risk of running to the end of the track or colliding with other trains, apparatus or vehicles on the improper track and colliding with road traffic such as cars and trucks at railroad-road crossings. In addition to safety concerns, control of trains is highly desirable for efficient and concentrated use of rail lines. In this regard one of the most important considerations is rail switching and rail switch positions. Improper switch positions can result in collisions, derailments, running through barriers, and running onto sidings causing loss of time in correcting improper train position. In addition to problems associated with improper switch position, incomplete switching can cause the same problems by leaving unacceptable gaps and distances between rails.
There has therefore been a continuing need for automatically detecting rail switch position. Some earlier devices for detecting switch position included electrical contacts between rails that closed when a switch was in a given position, and optical detection systems that detected the presence of a rail of the switch. These systems, while better than no automatic detection systems, had serious disadvantages. In particular contact switches were subject to failure due to wear and due to foreign matter such as soil, stone, and debris that became interposed between switch contacts. Electric contact switches had the further disadvantages of required insulated rails or at least insulated contacts, subject to insulation breakdown and the possibility of grounding out due to conductive articles or substances, e.g. water or even snow, in contact with the rail or contacts.
Another type of switch detector that has been tried is the photoelectric detector. Optical switch detectors were even more susceptible to interference from soil, debris, snow and ice by interference with the clear light path required for proper operation. Such detectors simply do not work well in an environment where dirt or snow can easily block a photodetector and photodetectors are usually sensitive to shock and vibration.
In the prior art, switch position detectors using simple self controlling flux generators having inductance-capacitance tank circuits as sensors were too unreliable for use because of tendency of flux levels and detection levels to drift thus resulting in no reliable standard to use as a basis for comparison when a train wheel entered the flux zone. Such drift resulted from a number of factors including temperature changes that altered component characteristics, presence of iron shavings or powder on the sensors, minor shifting of the sensor relative to the rail, and alteration of characteristics due to component aging. Such flux modification detectors further did not naturally contain reliable fail safe mechanisms indicating when they were operating improperly.
In addition such detectors are generally unreliable, for reasons previously stated, and are often costly and complex due to attempts to overcome the disadvantages previously described. A number of such patents in the somewhat related area of wheel detection, require both a field generator, such as a coil or permanent magnet and at least one detection coil that detects a change in flux density when a wheel flange approaches the coils. The use of both a field generator and a detection coil, or other multiple coil systems not only increases cost and complexity, the detectors are not as sensitive as desired. Examples of patents using multiple coils and or permanent magnets include U.S. Pat. Nos. Re 30,012; 3,697,745; 4,283,031; 4,524,932; 5,333,820; 5,628,479 and European Patent Application 0 002 609. Other systems, e.g. have employed the use of phase shift in an attempt to detect the presence of a train wheel. Such systems are subject to interference and are complex, e.g. as described in U.S. Pat. Nos. 5,395,078 and 3,721,821. A number of systems do not provide for compensation due to environmental factors and component aging, e.g. as described in U.S. Pat. No. 3,941,338, and still others use complex and unreliable circuitry where a microprocessor or other device is used to provide frequency generation that is then fed into a tank circuit, rather than relying upon a tank circuits own natural frequency. Examples of such patents include U.S. Pat. No. 6,371,417 and French patent application 80 25496.
Up to now, no known system has had the desired combination of properties of simplicity; reliability, including compensation; and fail safe detection of switch position, afforded by the apparatus and method of the present invention.